The need to find the relative elevations of points is extremely common, especially in construction work. This need is usually met by using a surveyor's level and levelling rod, or sometimes by a laser level and a sensor on a rod.
These methods are quite effective but in the case of a surveyor's level two people are required, one at the level and the other at the graduated levelling rod.
A laser level utilises a beam to form a horizontal plane and can be used by one person but it is quite expensive.
An object of the present invention is to provide a simple inexpensive device to enable the required elevation differences to be easily determined by one person.
The concept of the present invention is that a pair of vertical reflecting surfaces at right angles to each other, such as a pair of vertical mirrors at right angles to each other, or a right angled prism will reflect light back to its source, provided that the light source is not above or below the reflecting surfaces nor outside the field of view of approximately 90.degree..
If the mirrors have a horizontal mark on them, the horizontal plane containing that mark can be found by moving the observer's eye up and down until a reflection of the observer's eye coincides with the mark. This principle is already known, for instance in Australian Patent No. 539,881 (67092/81) where a telescope is used to assist in aligning the reflected image with the mark on the mirrors.
However, the device described in the abovementioned patent has a number of defects which make its use in practice difficult.
One defect is that it is often difficult to see the reflection of the observer's eye or telescope. One reason for this is that the reflected image of the observer's telescope is narrowed as the observer approaches the limit of the field of view near a vertical plane containing a mirror. The width of the reflected image of the telescope tends towards zero as this limit is approached. A second reason is that undesired reflections of adjacent walls and the like appear in the mirrors and makes it hard to see the desired reflection. This is compounded if the observer and his telescope are in subdued light and the walls or other objects are brightly illuminated.
The second problem is that it is difficult to maintain the mirrors in accurate vertical planes because uneven thermal expansion of the supporting tripod and frame upsets the vertical alignment. This is of particular concern because the observer will normally be at the levelling rod rather than the mirror device and hence may be unaware of the mis-levelment.
The present invention eliminates or greatly reduces the above problems.
Firstly, the difficulty of seeing the required reflection is reduced by using a bright light source at the observer's location. This light is positioned vertically above or below the position of the observer's eye. The observer moves his eye up and down until the reflection of the light is seen in the mirrors. The horizontal plane passing through the mirror is then halfway between the light and the observer's eye.
The advantage of using a light is that its reflection can easily be seen even if its subtends a very small angle at the observer's eye. This allows very small mirrors or a small prism to be used, each being typically 1 or 2 mm high and 10 or 20 mm wide. If the vertical aperture of the light is also small, say 2 or 4 mm, then it is possible to resolve elevation differences of a millimeter or so which is ample for most construction work.
Being able to use small mirrors or a small prism is not only economical but it also facilitates using an automatic levelling device to maintain them in a vertical plane.
The ease with which a light can be seen is dependent on the darkness of its background.
To facilitate seeing the reflected light the mirrors may be located in a housing which is very dark in color and which has slats around it to keep direct sunlight from illuminating the exterior of the container. Each pair of slats acts to make a heavily shaded area such as an observer would see looking into the interior of a black box. The slats also help to keep the interior cool.
The overall effect is that the housing appears to the observer as an extremely dark object. The reflected light shining out from inside can thus be easily seen, even when the observer is approaching the limit of the field of view. Another function of the housing is to shield unwanted reflections of brightly lit adjacent walls and the like which may be mistaken for the desired reflection or the light at the observer's location. A further precaution which may be taken to enable the reflection of the light easy to see is to provide a dark colored area immediately surrounding the light source and facing the mirrors.
The second problem to be overcome is to maintain the mirrors in accurate vertical planes regardless of thermal expansion over time.
In an embodiment of the present invention this is achieved by attaching the mirrors to a pendulum which has a damping arrangement to bring it to rest within a reasonable time.
The pendulum may be suspended by a system of threads, tapes or the like designed to allow it to oscillate freely in any direction without rotating.
The third problem with the device in the above-mentioned patent is that the eye of the observer has to be at the same elevation as the levelling device and that this may not be convenient on a sloping site. This difficulty is overcome in an embodiment of the present invention by the following means.
Instead of the observer being required to move his eye up and down the levelling rod to find the reflection of the light and hence the desired horizontal plane, he can keep his eye at the convenient height and move the light source up and down the rod until a reflection of it is seen at the mirror device.
The principle underlying this is that when the light and the eye are separated vertically, the horizontal plan passing through the mirrors and perpendicular to them is always half way between the light and the observer's eye. The reading on the levelling rod corresponding to the half-way point can be read directly using a special scale for this purpose.